Re: old 25hp mercury
Scaaty is right on! Great old motors, tons of power from about as compact a package as you will ever find. The 'Mark 25' name was just the model number, as it actually put out about 18 horses. You have also stumbled across that motors number one bugaboo - the weak lower unit. It is probably leaking oil due to rusted shafts, a common issue with old Mercs. You might be able to have the shafts welded and reground, or if it isn't too bad, you might be able to use a Speedisleeve to give a good seal surface. Might also want to inquire at Grubb's as to whether they could do the work for you if you can't improvise on a couple of special tools needed to take the unit apart. Don't try to do it yourself unless you have the 1965 & prior Merc manual...
Good luck!
- Scott
